1. Field of the Invention
The present invention relates to a plasma processing apparatus.
2. Description of the Related Art
Hitherto, as a plasma processing apparatus, a plasma processing apparatus, as shown in FIG. 11, has been known.
A conventional plasma processing apparatus has a matching circuit interposed between a high-frequency power source 1 and a plasma excitation electrode 4. The matching circuit is a circuit for obtaining impedance matching between the high-frequency power source 1 and the plasma excitation electrode 4.
The high-frequency power from the high-frequency power source 1 is supplied by a power-supply plate 3 to the plasma excitation electrode 4 through the matching circuit.
The matching circuit is housed in a matching box and the power supply plate 3 is housed in a house 21.
A shower plate 5 having a number of holes 7 formed therein is provided below the plasma excitation electrode (cathode electrode) 4, and a space 6 is formed by the plasma excitation electrode 4 and the shower plate 5. A gas introduction pipe 17 is provided in this space 6. Gas introduced from the gas introduction pipe 17 is supplied through the holes 7 of the shower plate 5 into a chamber 60 formed by a chamber wall 10. Reference numeral 9 denotes an insulator which insulates the chamber wall 10 from the plasma excitation electrode (cathode electrode) 4. Illustration of an exhaust system has been omitted.
Meanwhile, inside the chamber 60, a wafer susceptor (susceptor electrode) 8, on which a substrate 16 is placed, which acts also as a plasma excitation electrode is provided, with a susceptor shield 12 being provided around the susceptor electrode 8. The wafer susceptor 8 and the susceptor shield 12 are movable up and down by a bellows 11 so that the distance between the plasma excitation electrode 4 and the wafer susceptor 8 can be adjusted.
A second high-frequency power source 15 is connected to the wafer susceptor 8 through the matching circuit housed inside a matching box 14 and a shaft 13. The chamber and the susceptor shield 12 are at the same electrical potential in terms of direct current. In FIG. 11, reference numerals 61a and 61b each denote a resonance circuit, which acts as a band eliminator or a filter.
A case is considered in which, for example, high-frequency power of f.sub.1 =13.56 MHz is supplied to the plasma excitation electrode 4 and high-frequency power of f.sub.2 =100 MHz is supplied to the susceptor electrode 8.
The band eliminator 61b used for the susceptor electrode 8 is a series circuit of LC as shown in FIG. 11, and if EQU 2.pi.f.sub.1 =1/(L.sub.2 C.sub.2).sup.1/2
is set, a series resonance state is reached at a resonance frequency of f.sub.1, the impedance becomes a local minimum, and only a high-frequency wave of f.sub.1 can be selected and supplied to the susceptor electrode 8, making it possible to generate plasma in a state in which it is trapped between the plasma excitation electrode 4 and the susceptor electrode 8. With respect to f.sub.1 =13.56 MHz, the susceptor electrode 8 is nearly completely short-circuited to a ground.
FIG. 12 shows another conventional plasma processing apparatus.
In the plasma processing apparatus shown in FIG. 12, a shower plate 5 is not used, and the cathode electrode 4, which is a plasma excitation electrode, and the susceptor electrode 8 directly oppose each other. A shield 20 is provided around the rear surface of the cathode electrode 4. The remaining construction is the same as that of the plasma processing apparatus shown in FIG. 1.
In the conventional plasma processing apparatus, the impedance of a band eliminator is designed so as to generate plasma in a state in which it is trapped efficiently between the plasma excitation electrode 4 and the susceptor electrode 8. That is, the impedance is designed in such a manner as to be fixed so as to efficiently perform film formation.
In this regard, unlike during film formation, when cleaning of the chamber is performed, it is preferable that plasma be generated in such a manner as to be diffused over the entire chamber. In that case, the impedance with respect to the frequency f.sub.1 is preferably made a local maximum point. That is, it is preferable that a parallel resonance state be reached.
There is another case in which, as the value of f.sub.1, a frequency other than 13.56 MHz described above is used.
However, in the conventional plasma processing apparatus, since the impedance of a band eliminator is designed in such a manner as to be fixed in accordance with the frequency used, when cleaning of the chamber is desired to be performed or when another frequency is desired to be used, this must be performed after the band eliminator is replaced with another one.